Multifunctional magnetic rotator for micro and nanorheological studies

We report on the development of a multifunctional magnetic rotator that has been built and used during the last five years by two groups from Clemson and Drexel Universities studying the rheological properties of microdroplets. This magnetic rotator allows one to generate rotating magnetic fields in a broad frequency band, from hertz to tens kilohertz. We illustrate its flexibility and robustness by conducting the rheological studies of simple and polymeric fluids at the nano and microscale. First we reproduce a temperature-dependent viscosity of a synthetic oil used as a viscosity standard. Magnetic rotational spectroscopy with suspended nickel nanorods was used in these studies. As a second example, we converted the magnetic rotator into a pump with precise controlled flow modulation. Using multiwalled carbon nanotubes, we were able to estimate the shear modulus of sickle hemoglobin polymer. We believe that this multifunctional magnetic system will be useful not only for micro and nanorheological studies, but it will find much broader applications requiring remote controlled manipulation of micro and nanoobjects.     

Low-Temperature Oxidation, Hydrothermal Corrosion, and Their Effects on Properties of SiC (Tyranno) Fibers

The effects of oxidation in air and corrosion in high-temperature, high-pressure water on the mechanical properties of three commercially available amorphous Si-Ti-C-O (Tyranno) fibers with different oxygen contents (12%–18%) and diameters (8–11 μm) were investigated. The fibers were exposed to isothermal treatments at elevated temperatures and subsequently tested at room temperature. Structural changes in the fibers after oxidation and corrosion were also studied in order to understand better the degradation mechanisms of the fibers. Oxidation resulted in the formation of vitreous silica films and decreases of strength and Young's modulus of the fibers. Hydrothermal corrosion under 100 MPa water pressure started above 300°C and resulted in the formation of a carbon layer on the surface of the fibers. Dissolution of silica in water during the treatment was observed. Corrosion at temperatures above 400°C led to the formation of relatively thick carbon films which delaminated easily. It caused a decrease of strength and Young's modulus of the fibers. The hydrothermal method can be used for producing carbon coatings with thickness up to 2 μm on the surface of silicon carbide fibers. The degrading of the mechanical properties after oxidation and corrosion was controlled by the thickness of the oxide or carbon layer. Based on this fact, it is possible to predict changes in the mechanical properties from the oxidation data.     

Critierional evaluation of thermal destruction of corundum concretes

Conclusions The resistance of corundum concretes to thermal destruction was studied using the experimentally plotted deformation diagrams and the standard thermal cycling method. The correlation between the values of thermal shock resistance obtained according to these methods was established for the corundum concretes.It was established that as compared to the concrete produced using a binder based on the Talyum cement, the concretes incorporating a hydraulically hardening (water-setting) binder prepared from the VTs-70 grade high-alumina cement exhibit better resistance to thermal stresses. With increasing strength and density of the concretes, their thermal destruction occurs catastrophically (abruptly) since the elements of a stronger structure (the concretes produced using the Talyum cement-based binder) are incapable of effectively hindering the growth of a potential crack.Modification of the structure of the concrete by introducing Cr₂O₃ additive leads not only to an increased thermal shock resistance in the high-temperature range due to an increased critical crack length, but also to an increased resistance ot crack displacement (opening).Translated from Ogneupory, No. 5, pp. 3–8, May, 1988.     

Kinetics of aluminum extraction from Ti3AlC2 in hydrofluoric acid

Herein we report on the influence of particle size, time and temperature on the kinetics – quantified by X-ray diffraction – of the selective extraction of Al from the ternary layered transition metal carbide, Ti₃AlC₂, when powders of the latter are immersed in hydrofluoric acid. Transmission and scanning electron microscopy, energy-dispersive X-ray spectroscopy and thermogravimetric analysis were also used to characterize the resulting powders. Increasing the temperature and immersion times, and decreasing the Ti₃AlC₂ particle size, led to faster conversion of Ti₃AlC₂ to its 2-D Ti₃C₂ counterpart. Arch-shaped edges at the ends of some Ti₃C₂ layers resembled graphene, corroborating the single-sheet structure of exfoliated Ti₃C₂. The removal of water and/or OH surface groups from Ti₃C₂ using drying in vacuum was also attempted.